A training sequence is a series of a priori known symbols which are transmitted at predetermined times from a signal transmitter to a signal receiver. Such sequences have long been used to adjust the operation of signal dispersion compensation apparatus in the receiver, such as equalizers, echo cancellers and the like.
Signal dispersion, such as echoes or signal "ghosts", is an inherent problem in communications systems and the severity of the problem can vary with the system application. For example, signal ghosts or echoes frequently exist in conventional television transmission systems which are highly objectional to the viewer and which can render a high definition television (HDTV) signal unintelligible. Accordingly, compensation for signal dispersion is highly desirable in conventional television systems and, indeed, is required in HDTV and in many other communications applications. Moreover, while signal dispersion compensation apparatus, such as equalizers and cancellers, provides satisfactory compensation, knowledge of the signal dispersion characteristics, e.g., amplitude, delay and phase, is useful for the adjustment of such apparatus during system start-up and operation.
Prior art techniques exist which can determine the characteristics of signal dispersion. Such techniques typically transmit specific signals, such as pulses or training sequences, which are detected and analyzed in the receiver. These techniques provide satisfactory estimates of the signal dispersion characteristics in applications wherein the dispersion is large in amplitude and short in duration. However, the prior art techniques provide inaccurate results when the signal dispersion is small in amplitude, particularly in the presence of channel noise, jitter or similar impairments, and provide ambiguous results when the signal dispersion is long in duration. These shortcomings have hindered development of communications systems requiring ever-more precise signal dispersion compensation and more precise determination of the signal dispersion characteristics.
Most recently, as disclosed in a pending application to Chao et al., Ser. No. 443,772, filed Nov. 30, 1989, and assigned to the present assignee, a technique which overcomes the aforesaid problems of the prior art has been developed which transmits two different training sequences wherein each such training sequence includes a different number of symbols. This technique, while providing greater accuracy than those of the previous prior art, requires a time duration for the transmission of the two different training sequences which is not always available. Therefore, a technique which provides greater accuracy in the determination of small-amplitude and long-duration signal dispersion and which requires less time duration would be desirable.